Detection of far-infrared (C-13)O line emission

The authors have detected far-infrared 13CO line emission toward the core of Orion-KL. About 10 - 30 M_sun; of dense and warm (T ≥ 200K) gas are required to account for the 151 μm J = 18 → 17 13CO line flux. At line center the J = 18 → 1713CO line is about 25 times weaker than the 17 → 16 12CO line. The 13CO and 12CO far-infrared lines cannot come from a single, optically thin component of gas. The emission could originate either from an optically thick source of temperature near 200K or, more likely, from several components of gas.